Remove the allowed_updates key

This isn't a valid key, and according to claude it's not possible to configure
it to get only security updates (I'm too lazy to read the docs). I suppose this
means we will now get pull requests for any updated dependencies, but maybe
that's useful; we'll see how annoying this gets.

.github/dependabot.yml

@@ -4,6 +4,3 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
-    allowed_updates:
-      - match:
-          update_type: "security"